Overhead bridge cranes which travel on their wheels along spaced apart, generally parallel rails are subject to a variety of uneven forces which adversely affect the traction and efficient running of the cranes. Such forces typically cause skewing of the crane on the rails and high stress on the rails and the components of the cranes. Bridge crane drives are generally of two types. Either a cross shaft drive is used in which the opposite drive wheels engaging each of the parallel rails are mechanically connected together, so that under all crane traveling conditions the two wheels operate at the same rotational speed. The second type of drive is an independent drive at each opposite wheel of a pair of drive wheels which operate at the same rotational speed while the crane is operating in a desired, straight-line manner parallel to the rails. However, if the crane is subject to uneven loads or forces, including forces which may cause skewing, the rotational speed at either one of the independently driven wheels may decrease or increase to cause skew and exacerbate, or prevent recovery from, a skewed position of the crane.
Where two pairs of crane wheels are driven, as is common on larger bridge cranes, two cross shaft type drives may be used. However, in a double cross shaft drive, if a wheel of one pair of cross shaft connected wheels is subjected to forces which cause it to attempt to change speed, the difference in energy between the two cross shaft connected wheels is stored in the mechanical system of the two connected wheels, such as in shaft and gear members, bearings, and support and bolt members of the drive system. The storage of energy is more of a problem in a double cross shaft drive system, as compared to a single cross shaft drive system, because the former will not as readily change speed or position as the latter to avoid storing of energy in the drive system. The energy stored as described above in a double cross shaft drive system is subsequently released intermittently upon any momentary light load occurrence or traction loss such as rolling over a bump, hitting a joint, or joggling of the wheels as a result of a load change. This intermittent energy release causes further uneven running of the crane to result in its again storing energy and the perpetuation of a highly undesirable situation.
The other type of common drive system for large cranes where more than two wheels are driven is the driving of each wheel of each of several pairs of wheels with a separate, independent drive for that wheel. Independent drive systems have the well-known problem, when one of the independently driven wheels of a crane is subjected to unbalanced forces relative to other driven wheels such as unbalanced inertia forces of the swinging load carried by the crane trolley, to slow or increase the speed of the crane and cause it to quickly go into skew. Cranes driven with independent drives at each wheel require constant correction of a skewed condition of the crane, and are continuously subjected to undesirable, unbalanced stressing conditions.